Julia package development

Work on a warm Julia session and verify each edit as you make it. The cost of a fresh process is time-to-first-execution paid once per session, so keep one alive and check every change against it. An agent that verifies per step writes better Julia: genuinely non-allocating where asked, and more likely to get the task right first time.

Warm session

Start one session and reuse it. Under Claude Code the AgentREPL.jl plugin gives a warm MCP session over STDIO, with no open port. Its eval tool is synchronous, so a call returns when evaluation finishes. A PostToolUse hook nudges you to reload after editing a .jl file.

Point the session at the project with activate, then instantiate. Redefining state or types Revise cannot reload means restarting the worker with reset.

Revise

Call the revise tool after editing a .jl file; session state is kept. Struct and type edits now reload on Julia 1.12+ with Revise 3.16+ and revise_structs = true, but objects built before the edit keep the old type, so a session holding such state is still stale.

Still needs a fresh worker:

macro and generated-function changes
a struct reached only through a type alias or global binding
corrupted or polluted state; restart rather than debug it

Testing with TestItemRunner

Write tests as @testitem "name" begin ... end. Each item runs in a fresh module, so runs stay isolated even in a shared session.

Run using TestItemRunner once, then filter to what you changed:

# by file
@run_package_tests filter=ti->contains(ti.filename, "foo.jl")
# by name
@run_package_tests filter=ti->contains(ti.name, "edge")
# by tag
@run_package_tests filter=ti->!(:slow in ti.tags)

Run the narrowest filter that covers the change while iterating. Run the full suite on a fresh session before finishing.

Hot versus cold

Run most work on the warm session. Fall back to a fresh process for:

final verification before commit
precompilation, load-time, or TTFX work
struct, type, or const redefinitions Revise cannot reload
stale state

Dependencies

Use Pkg.update() rather than editing Project.toml by hand. You can Pkg.add then using in a live session, but restart the worker afterwards if the new package changes what is already loaded.

Formatting and quality

Format with JuliaFormatter, usually through a pre-commit hook. Keep Aqua.jl tests in test/ for package-quality checks. Follow the project style: 80-character lines, no trailing whitespace, type-stable code. Check type stability with @code_warntype and watch for Any.

Docstrings

Use DocStringExtensions templates, and mind template expansion:

@doc "..." (regular string) lets templates expand
@doc raw"..." does not expand templates; use only without them
for templates with LaTeX, use @doc """...""" and escape backslashes (\\int, not \int)

@doc "
$(TYPEDSIGNATURES)

Brief description.

# Fields
$(TYPEDFIELDS)
"
struct MyType
    "Field description"
    field::Int
end

Keep interface docstrings to a line or two and cross-reference related methods with a See also: [`other`](@ref) line rather than repeating content.

When to use this skill

Use it when developing, testing, documenting, or profiling Julia packages. Project-specific architecture and domain knowledge belong in the project CLAUDE.md, not here.

julia-development